Two-dimensional magnetic recording (TDMR) is a recording architecture intended to support storage densities beyond those of conventional recording systems. TDMR reads from one or more adjacent tracks to decode the signal from a target track. The gains achieved from TDMR come primarily from joint processing of multiple readback signals using powerful coding and signal processing algorithms which allow data bits to be stored more densely on a magnetic storage medium (e.g., disk). In a traditional disk architecture with a single read head, collection of multiple readback signals to facilitate the joint processing would require additional disk rotations. To circumvent this problem, TDMR disk drives may use multiple readers assembled on the same support arm, typically referred to as a slider, thus restoring traditional read service times.
Densely packed disks utilize narrower tracks. To achieve narrower tracks, one may use shingled writing, where each sweep of the write head overlaps a certain percentage of the previous track. Shingled writing may be used without two-dimensional readback at lower densities. However, as the shingling gets more aggressive, one-dimensional codes and detectors will eventually no longer be able to handle intertrack interference (ITI) coming from adjoining tracks. In one-dimensional channels, ITI leads to a performance loss. With two-dimensional channels, ITI assists in making a data decoding decision (i.e., logical assignment) for a given bit.
One disadvantage of using a multi-reader approach is that there is an inherent offset (i.e., delay) between read signals obtained from the respective readers due, at least in part, to the physical distance—down-track and/or cross-track—separating the readers. The physical distance between readers causes the offset between read signals, which needs to be compensated for efficient TDMR signal processing. Although manufacturers may provide the physical distances between the multiple readers, actual offset between read signals obtained from the respective readers can vary based on many factors, including, but not limited to, temperature, fly-height variation, read head misalignment, physical damage, physical vibration, skew angle, etc. Because of these various factors, estimating the actual offset between readers can be challenging.